The most use I see for testing legacy code you have to extend is to test/document your understanding of it. This are 'learning' tests as it were.
If things later get funky, you can fall back on these tests to see what you understood of the legacy code back when you wrote it. And it will help you when someone "maintained" the legacy code to do something different.
On the other hand in order for these tests to make sense later on, these must be limited to the interactions with the stuff being added/modified. Or at least the tests for the basic functionality should be separated.